The present invention relates to a zoom lens, and specifically to a zoom lens having a variable magnification ratio from about 2.5 to 6, which is appropriate for an electronic still camera, video camera, or similar device, using a solid-state image sensor such as a CCD, or the like.
Recently, the development of software to process image data is remarkable with the technological advance or the spread of a personal computer, and a demand for an electronic still camera, used for image reading into a personal computer, increases.
As a zoom lens for a solid-state image sensor, conventionally, the zoom lens of a 3-group composition or 4-group composition is disclosed. As a zoom lens for a video camera, it has a tendency toward high variable magnification, and the zoom lens of 4-group composition is widely used. As a zoom lens for an electronic still camera, high resolution and high image quality are required, and the zoom lens having about 3 times-variable magnification ratio, which is a ratio of the focal length at the wide angle end and that at the telephoto end, is widely used, and 3-group composition is also disclosed for the zoom lens having about 3 times-variable magnification ratio.
Conventionally, the following are disclosed for the zoom lens having 3-group composition: Japanese Patent Publication Open to Public Inspection Nos. 79716/1989, 291515/1990, 89309/1991, 106512/1992, 307509/1992, 203875/1993, 271788/1996, etc.
However, in these conventional examples, there is a problem that the overall length of the lens is long in proportion to its variable magnification ratio in a zoom lens in which zooming is conducted by moving the second lens group and the third lens group, in a 3-group lens composed of positive, negative, and positive lens groups. Further, distortion at the wide angle end exceeds about -5%; the distortion is small, but the overall length of the lens is too large; or correction of the spherical aberration or astigmatism is insufficient.
Further, in the zoom lens whose aberration is sufficiently corrected, in order to have about 3 times-variable magnification ratio, cost is higher because it is necessary that the number of component lenses is 11-12, or many aspherical glass lenses whose production cost is relatively high, are used.
Further, in an example in which priority is given to cost reduction, all lenses are made of plastic, and variable magnification ratio is made large, therefore, there are problems in that the overall length of the lens is extremely increased, and it is necessary to compensate for the influence of temperature variation by any method except for lens, or because many aspherical surfaces are used, the sensitivity of performance deterioration, caused by eccentricity of the lens, is high, and assembling becomes difficult.